If you can not download an audio file, video or photo that has sent you a friend via WhatsApp, or if you can not upload a movie to send it, first verify that your smartphone is connected to the Internet.
If you're using a WiFi network, it can sometimes happen that the router signal is loud and clear, but the network is temporarily out of order. In such cases, even if there seems to be covering, in reality you're offline. Make sure that everything is working properly instead.
After checking that there is free space on the SD card. Go to Settings> Memory, and there read what is the amount of memory that you have and what is still available.
Storage left
If there is little, you have to free up a bit '. A good idea is, for example, delete photos and movies and save them online. In this article we recommend some good facilities, easy to use, to keep all your shots on the cloud.
Another possibility is that, for some reason, the SD card is not writeable (perhaps because it is in read only mode: try to copy a file and see if you can do it.
If the space is there but the files can not be copied, drag all those who have the card in your computer (so you will not lose anything), and then format it. Forgive if you want the files in SD and checks that it is now possible to download files from WhatsApp.